Artificial dental restorations are typically made of one or more layers of dental porcelain applied to a metal substrate. Typically the porcelain is applied to a cast metal base. The cast metal base is relatively thick and is made by casting the molten metal using the well known lost wax technique. Dental porcelain is applied to the cast metal base and the entire structure is baked at high a temperature. The thickness of the cast metal base is typically 0.3 to 0.5 mm. This thickness can be disadvantageous because it minimizes the permissible thickness of the translucent porcelain. In addition, the success of the restoration depends to a great extent on the skill of the practitioner in preparing the cast metal base.
An alternative dental restoration involves forming a crown of dental porcelain on a foil made of a noble metal such as platinum. (An improved metal foil for metal-porcelain dental restorations of this type is disclosed in co-pending application Ser. No. 049,119 filed May 13, 1987.) Generally, these foils can have a thickness of 200 microns or less. The thin platinum foil is shaped to a replica, or die, of the prepared tooth and the dental porcelain is then overlaid onto the foil which supports the porcelain during the subsequent firing cycles. It has been noted that this procedure is extremely technique-sensitive, requiring skillful and meticulous processing. As observed in U.S. Pat. No. 4,585,417 issued Apr. 29, 1986 to Sozia et al., proper and accurate shaping of the platinum foil over the replica is extremely difficult, and these difficulties frequently lead to poorly fitted crowns.
Various techniques have been proposed for fitting metal foils over a die of a prepared tooth. These techniques typically include some sort of swaging step to get as close a fit as possible between the foil and the die. Thus, U.S. Pat. No. 4,459,112 issued July 10, 1984 to Shoher et al. discloses a method of forming a crown wherein a circular foil is placed over a die and carefully folded into at least three pleats. The pleats are folded in an overlapping formation, and either the foil is swaged or pressure is applied by hand. Another technique as set forth in Trends & Techniques in the Contemporary Dental Laboratory, Vol. 3, No. 7, September, 1986, pp. 21-25, involves carefully cutting and folding the foil around the die, then "swedging" the foil covered die by surrounding it with a putty-like material and subjecting that material to hydraulic pressure via hammer blows to a plunger. These prior art methods are laborious and time consuming. Further, they require great skill on the part of the clinician to fit the foil completely uniformly around the die. In addition, standard dental dies are made of gypsum and like materials which can be cracked or otherwise damaged when subjected to swaging procedures. Even if the underlying die is not completely destroyed, sharp edges and other fine details of the die structure can be marred or altered by the swaging process.